The present invention concerns the well known process (hereafter “Process”) for the cryogenic separation of an air feed wherein:
(a) the air feed is compressed, cleaned of impurities that will freeze out at cryogenic temperatures such as water and carbon dioxide, and subsequently fed into an cryogenic air separation unit (hereafter “ASU”) comprising a main heat exchanger and a distillation column system;
(b) the air feed is cooled (and optionally at least a portion condensed) in the main heat exchanger by indirectly heat exchanging the air feed against at least a portion of the effluent streams from the distillation column system;
(c) the cooled air feed is separated in the distillation column system into effluent streams including a stream enriched in nitrogen and a stream enriched in oxygen (and, optionally, respective streams enriched in the remaining components of the air feed including argon, krypton and xenon); and
(d) the distillation column system comprises a higher pressure column and a lower pressure column;
(e) the higher pressure column separates the air feed into effluent streams including a high pressure nitrogen stream withdrawn from the top of the higher pressure column, and a crude liquid oxygen stream withdrawn from the bottom of the higher pressure column and fed to the lower pressure column for further processing;
(f) the lower pressure column separates the crude liquid oxygen stream into effluent streams including an oxygen product stream withdrawn from the bottom of the lower pressure column, and a low pressure nitrogen stream withdrawn from the top of the lower pressure (and often a waste nitrogen stream which is withdrawn from an upper location of the lower pressure column); and
(g) the higher pressure column and lower pressure column are thermally linked such that at least a portion of the high pressure nitrogen is condensed in a reboiler/condenser against boiling oxygen-rich liquid that collects in the bottom (or sump) of the lower pressure column and used as reflux for the distillation column system.
More specifically, the present invention concerns the known embodiment of the above-described Process wherein, in order to provide the refrigeration necessary when at least a portion of the product is desired as liquid, refrigeration is extracted from liquefied natural gas (hereafter “LNG”) by feeding nitrogen from the distillation column system to an insulated liquefier unit (hereafter “LNG-based liquefier”) where it is liquefied. If at least a portion of the liquid product desired is liquid oxygen, at least a portion of the liquefied nitrogen is returned to the distillation column system (or optionally the main heat exchanger). Otherwise, the liquefied nitrogen is withdrawn as product.
Typical of LNG-based liquefiers, the nitrogen is compressed in stages and cooled between stages by indirect heat exchange against LNG. If the compression is performed with a cold-inlet temperature, the LNG will also be used to cool the feed to the compressor as well as the discharge by indirect heat exchange. Examples of LNG-Based liquefiers can be found in GB patent application 1,376,678 and U.S. Pat. Nos. 5,137,558, 5,139,547 and 5,141,543, all further discussed below.
The skilled practitioner will appreciate the contrast between an LNG-based liquefier and the more conventional liquefier where the refrigeration necessary to make liquid product is derived from turbo-expanding either nitrogen or air feed.
An LNG-based liquefier is typically oversized to accommodate a projected increase in demand of liquid products after the initial years of operation. This is particularly true for liquid nitrogen since the demand for liquid nitrogen out of any particularly ASU often grows faster than the demand for liquid oxygen above the base load of liquid oxygen for which the plant is designed. A problem with this oversizing approach however is the incremental capital cost incurred does not begin to pay off until the projected demand increase is actually realized (if at all). Furthermore, capital costs are particularly sensitive for LNG-based liquefiers since, as opposed to conventional liquefiers which are typically located near the customers of the liquid products, LNG-based liquefiers must be located near an LNG receiving terminal and thus incur a product transportation cost penalty.
To address this problem, the present invention is a system to increase the capacity of the LNG-based liquefier comprising a supplemental compressor that is separate and distinct from the auxiliary compressor(s) contained in the LNG-based liquefier. This allows the supplemental compressor and the associated heat exchange equipment to be purchased and installed when the projected demand increase is actually realized, if at all. In this fashion, the incremental capital that would have otherwise been spent on oversizing the LNG-based liquefier from the start does not get spent until it is actually needed. Another benefit of the present invention is that the capacity increase is primarily directly toward the ability to produce liquid nitrogen which, as noted above, will often have a demand that grows faster than the demand for the liquid oxygen from the plant.
The skilled practitioner will appreciate that, as an alternative to the present invention, the capacity of an LNG-based liquefier can be increased by adding a dense fluid expander. However, only modest capacity increases can be achieved in this manner.
GB patent application 1,376,678 (hereafter “GB '678”) teaches the very basic concept of how LNG refrigeration may be used to liquefy a nitrogen stream. The LNG is first pumped to the desired delivery pressure then directed to a heat exchanger. The warm nitrogen gas is cooled in said heat exchanger then compressed in several stages. After each stage of compression, the now warmer nitrogen is returned to the heat exchanger and cooled again. After the final stage of compression the nitrogen is cooled then reduced in pressure across a valve and liquid is produced. When the stream is reduced in pressure, some vapor is generated which is recycled to the appropriate stage of compression.
GB '678 teaches many important fundamental principles. First, the LNG is not sufficiently cold to liquefy a low-pressure nitrogen gas. In fact, if the LNG were to be vaporized at atmospheric pressure, the boiling temperature would be typically above −260° F., and the nitrogen would need to be compressed to at least 15.5 bara in order to condense. If the LNG vaporization pressure is increased, so too will the required nitrogen pressure be increased. Therefore, multiple stages of nitrogen compression are required, and LNG can be used to provide cooling for the compressor intercooler and aftercooler. Second, because the LNG temperature is relatively warm compared to the normal boiling point of nitrogen (which is approximately −320° F.), flash gas is generated when the liquefied nitrogen is reduced in pressure. This flash gas must be recycled and recompressed.
U.S. Pat. No. 3,886,758 (hereafter “U.S. '758”) discloses a method wherein a nitrogen gas stream is compressed to a pressure of about 15 bara then cooled and condensed by heat exchange against vaporizing LNG. The nitrogen gas stream originates from the top of the lower pressure column of a double-column cycle or from the top of the sole column of a single-column cycle. Some of the condensed liquid nitrogen, which was produced by heat exchange with vaporizing LNG, is returned to the top of the distillation column that produced the gaseous nitrogen. The refrigeration that is supplied by the liquid nitrogen is transformed in the distillation column to produce the oxygen product as a liquid. The portion of condensed liquid nitrogen that is not returned to the distillation column is directed to storage as product liquid nitrogen.
EP 0,304,355 (hereafter “EP '355”) teaches the use of an inert gas recycle such as nitrogen or argon to act as a medium to transfer refrigeration from the LNG to the air separation plant. In this scheme, the high pressure inert gas stream is liquefied against vaporizing LNG then used to cool medium pressure streams from the air separation unit (ASU). One of the ASU streams, after cooling, is cold compressed, liquefied and returned to the ASU as refrigerant. The motivation here is to maintain the streams in the same heat exchanger as the LNG at a higher pressure than the LNG. This is done to assure that LNG cannot leak into the nitrogen streams, i.e. to ensure that methane cannot be transported into the ASU with the liquefied return nitrogen. The authors also assert that the bulk of the refrigeration needed for the ASU is blown as reflux liquid into a rectifying column.
U.S. Pat. Nos. 5,137,558, 5,139,547, and 5,141,543 (hereafter “U.S. '558”, “U.S. '547”, and “U.S. '543” respectively) provide a good survey of the prior art up to 1990. These three documents also teach the state-of-the-art at that time. In all three of these documents, the nitrogen feed to the liquefier is made up of lower pressure and higher pressure nitrogen streams from the ASU. The lower pressure nitrogen stream originates from the lower pressure column; the higher pressure nitrogen stream originates from the higher pressure column. No direction is given as to the ratio of the lower pressure to higher pressure nitrogen streams.
There is little new art in the literature since the early 90's because the majority of applications for recovery of refrigeration from LNG (LNG receiving terminals) were filled and new terminals were not commonly being built. Recently, there has been resurgence in interest in new LNG receiving terminals and therefore the potential to recover refrigeration from LNG.